Turboprop engines are a type of aircraft power plant that use a gas turbine to drive a propeller.
Modern turboprop propulsion systems generally operate constant speed propellers. The pitches of the blades of the propellers are varied to absorb the supplied shaft power.
In typical turboprop installations, the pilot is supplied with a limited range within which he is permitted to vary propeller speed (by varying the pitches of the propeller blades) in order to increase turboprop efficiency. This increase in efficiency tends to reduce fuel burn and emissions. An amount to vary the propeller speed tends to be deduced manually, e.g. from charts supplied by the aircraft manufacturer.
It tends to only be practical to adjust propeller speed manually in a periodic fashion, and periodically on long steady flight segments. This is to avoid excessive pilot workload.
Furthermore, other than that provided by the pilot, there is typically no integration of the propeller, powerplant and airframe. Thus, in unmanned aircraft there is typically no method of increasing turboprop efficiency by varying propeller speed.